Study On Reverse Martensite Transformation And Mechanical Behavior Of Fe-Mn-C Based Alloy

High manganese austenitic steel is an alloy with exceptional properties,characterized by high tensile strength,excellent work hardening ability,and good ductility.These outstanding features make it a promising material for a wide range of applications in the automotive and industrial fields.However,its low yield strength limits its use in some situations.In addition,the mechanisms and pseudo-elasticity of its non-thermally activated martensitic reverse phase transformation have yet to be fully explored.This study investigates the mechanism of stressinduced non-thermal martensitic transformation and its inverse in Fe-Mn-C alloys with low,medium,and high stacking fault energies.The effects of Cu-rich and Mo-rich nanophases,as well as strain rate,on the mechanical behavior of these alloys are also examined.Using the theory of back stress,a dislocation model is proposed to explain the inverse transformation of non-thermal martensite.The relationship between microstructure and macroscopic mechanical properties is analyzed,providing a theoretical basis and experimental reference for understanding the non-thermal martensitic transformation mechanism and optimizing the mechanical behavior of austenitic high manganese steels.(1)Through cyclic loading-unloading experiments and microstructure characterization,the microstructure evolution and pseudoelastic strain of Fe-18Mn-0.5C and Fe-18Mn-0.5C-3Cu alloys within the range of 0 to 0.1 true strain were quantitatively compared.The results showed that the Fe-18Mn-0.5C alloy exhibited better pseudoelasticity than the Fe-18Mn-0.5C-3Cu alloy,as it could undergo non-thermoelastic martensitic phase transformations during loading-unloading cycles.The relationship between incomplete dislocation behavior and mechanical response was analyzed and discussed based on the experimental results and back stress theory under the mechanism of martensitic phase transformation/reverse transformation.(2)Based on the mechanism of non-thermoelastic martensitic reverse transformation,the influence of Cu-rich nanoprecipitates on the mechanical behavior of the Fe-18Mn-0.5C-3Cu alloy was analyzed.The evolution of back stress and effective stress in the alloy was quantitatively studied based on the stress distribution.By comparing the macroscopic mechanical behavior and microstructure differences between the solution-treated and aged states,it was found that Cu-rich nanoprecipitates mainly increased the material strength and pseudoelasticity by enhancing the back stress.(3)The effect of strain rate and Mo-rich nanoprecipitates on the mechanical properties of a higher energy barrier Fe-20Mn-5Mo-1C alloy was explored.Through experimental results and theoretical analysis,it was found that the strain rate mainly changed the deformation mechanism of the material by affecting the critical shear stress for dislocation slip and twinning.Under low-speed tension,the material was mainly dominated by dislocation slip,while under high-speed tension,the material was mainly dominated by twinning.In addition,by comparing the macroscopic mechanical behavior and microstructure differences between the solutiontreated and aged states,it was found that Mo-rich nanoprecipitates effectively hindered dislocation slip and increased the material strength by enhancing the back stress.However,Morich nanoprecipitates also caused stress concentration,leading to a significant reduction in material ductility.